We talked about transfer cases and their problems before here on LifeOnFour. Since then, we got a number of emails asking us to cover how all-wheel drive and how 4-wheel drive drivetrains actually work. You seek, we deliver. Today we are talking about how a part-time 4-wheel drive works and what it is!
How Does A Part-Time 4-Wheel Drive System Work?
The engine distributes the power to the front and rear axles via a central transfer case, as with other all-wheel-drive systems. In contrast to the permanent 4-wheel drive, however, there is no differential in the transfer case, but only a clutch, which only knows the two operating states “open” and “closed”, no slip. This means that the front and rear axles can either be engaged or completely separated. In the usual design, the rear axle is driven directly, while the front axle can be switched on – the other possibility of driving the front axle directly and switching on the rear one if necessary is not common, but with appropriate conversion measures, some transfer cases can be modified in this way.
In almost all cases, the transfer case also includes the reduction gear, which, if necessary, reduces the roadworthy transmission by a factor of 1.5-2 (up to 4 depending on the model, or higher after appropriate conversions). Based on a factor of 2, this achieves twice the torque at the wheel at half the wheel speed and makes climbing capabilities of 45 degrees and (mathematically) more possible in the first place. It is also often spoken of as 12 gears or 10+2, which is technically correct, but usually cannot be used in this way when driving, since only very few models have synchronized gear reduction. So you can’t switch between high and low gear ratios while driving.
This patent and its pictures clearly show how the drivetrain of a part-time 4-wheel drive vehicle is constructed. Check it out for more details!
How Do You Switch Between 2-Wheel Drive and 4-Wheel Drive?
In the old days, there were only purely mechanical shift options in the form of a second shift lever behind or next to the gear shift lever, but over time this was replaced by electromechanical designs. On the current models, there is a rotary switch or button set somewhere in the dashboard, and the actual actuation of the transfer case is done via servo motors. Usually, the settings 2H, 4H, 4L, and N are possible. As is not difficult to guess, the first number indicates the number of driven wheels, “H” is the gear ratio for high speeds, and “L” is the gear ratio for slow but powerful Off-road driving. There is no 2L option as there is no point in using low gears with 2-wheel drive. The operating manual usually advises against this, as the highest possible torque is then released on an axle. “N” is neutral, but this is not always present. At best, it is necessary for vehicles with automatic transmission in order to be able to tow them without damaging the automatic transmission.
Can you switch from 2wd to 4wd while moving?
Yes, you can switch between 2-Wheel Drive and 4-Wheel Drive while driving, however, it should be done at speeds below 60 mph or 100 kph. Make sure you consult your owner’s manual for further details as all vehicles are different. While most vehicles can shift between 2wd and 4wd while driving, not all vehicles are like this! Be careful and shift the drive mode while standing still if you are not sure!
Can you switch from 4Hi to 4Lo while moving?
No!! Switching into “low” gear ranges requires moving below 5 mph or ideally when standing still. You must also switch your transmission into N (neutral).
Do you have to be in neutral to switch from 2wd to 4wd?
It depends on the vehicle. Older 4-Wheel Drive vehicles do demand that you shift to neutral when shifting between the driving modes. Consult your owner’s manual if you are in doubt. Failing to properly operate your drivetrain can result in major damage.
How often should you engage 4-wheel drive?
Whenever you really need it and the terrain demands it. Avoid using it on surfaces with a solid grip like tarmac and light gravel roads. Engage 4WD whenever you need it to tackle rough terrain.
Disadvantages Of Part-Time 4-Wheel Drive
The crucial point of this type of drive is mentioned above. There is no center differential, and front and rear axles always rotate at exactly the same speed in 4-wheel drive. This has no disadvantages as long as you only drive straight – in curves, however, the front axle travels a longer distance than the rear axle, so the car should also turn faster. However, due to the fixed connection of the front and rear cardan shafts in the transfer case, it cannot. This has two decisive disadvantages for the driving dynamics: the front axle in the turning radius is bigger and it also causes the vehicle to understeer heavily. The force feedback and vibrations in the steering wheel are also very high. Furthermore, the brake force distribution is suddenly no longer front-heavy, but distributed equally to both axles – the rear axle, therefore, tends to overbrake. In summary: switchable 4-wheel drives and curves do not get along.
So you have to think about when you really need the 4-wd traction benefits and when you don’t. It should also be noted: You definitely do not need them on firm ground such as tarmac, because all 2WD vehicles can also move there without any problems. At the same time, using 4-wd on solid ground can result in serious damage to the drivetrain if the tension in the drivetrain that is caused by the speed differences cannot be compensated for by wheel slip. If the wheels stick too well, at some point or another, drivetrain components will give out. – There are a lot of expensive drivetrain parts, from the transfer case to the suspension, axle joints, the drive and propeller shafts and the wheel hubs. Some manufacturers have provided an emergency release device for such cases, which jumps over (loudly) if the load is too high, similar to a toothed slipping clutch – but this is counterproductive in terms of the best possible power transmission and off-road capability since the all-wheel drive is supposed to hold up in such situations.
Benefits Of Part-Time 4-Wheel Drive
As is often the case in life, it is a question of need and price. A switchable part-time 4-wheel drive is initially cheaper than a permanent one. In contrast, it does not serve to improve driving dynamics but merely aims to get through difficult situations. You don’t need a center differential for this – because that would be the first thing to be locked anyway, so as not to let all the power go to waste. On the loose ground and off-road, the switch-on 4-wheel drive is at least as good and useful as any other – and you may not use it on the road, but you don’t necessarily need 4-wheel drive, which the majority of cars with only 2-everyday. During the winter and “winter” road conditions, where the knowledge that “I have all-wheel drive” suggests superiority over “normal” cars. In fact, with the 4-wheel drive, this only refers to starting and getting through the worst parts of winter, most of the time, the driving dynamics are rather worse – which is difficult to convey to most drivers.
What is the difference between full-time 4wd and part-time 4wd?
Full-time 4-Wheel Drive vehicles transfer the power from the engine and transmission to all 4 wheels of the vehicle all of the time. There is no option of switching to 2-Wheel Drive.
Part-time 4-Wheel Drive vehicles on the other hand allow the driver to switch between 4-Wheel Drive and 2-Wheel Drive when they please do so. Usually, 2-Wheel Drive will be reserved for driving in cities and on the tarmac. And 4-Wheel Drive will be used for serious off-roading.
Part-time 4-Wheel Drive vehicles are often looked down upon amongst the 4WD owners and fanatics. However, for a regular Joe that likes to visit rough off-road trails every now and again, a part-time 4WD vehicle is a perfect choice. Vehicles with part-time 4WD can do it all. They can be great on the highway and drive perfectly around sharp corners, but on the other hand, they can handle almost all the terrain a serious full-time off-roader can.